best antenna

[trigger]

Hi ab cd, I must have missed your calculation for the element length of 114mm in post #1811. I thought with a VF of 0.83 we would be looking at an element length of 124mm. Is the difference due to the effect of the matching element

[abcd567]

Hi ab cd, I must have missed your calculation for the element length of 114mm in post #1811. I thought with a VF of 0.83 we would be looking at an element length of 124mm. Is the difference due to the effect of the matching element

Length is calculated by standard formula:
Element length = VF x 1/2 wavelength = 0.83 x 1/2 x 275 = 114 mm

[trigger]

Length is calculated by standard formula:
Element length = VF x 1/2 wavelength = 0.83 x 1/2 x 275 = 114 mm

Ooooops ! I obviously made a mistake a few months ago when I worked out the element lengths and wrote them down. I referred back to those notes this morning. This mistake will invalidate all the CoCo experiments I've done in the past

[hikeofyourlife]

I'm wondering how much the outer jacket brings down the VF, since you're using the outside of the shield as the actual antenna elements and not the inside of the coax. I'm thinking the twin-lead tuning method works because the VF is not 83 percent anymore and you're actually correcting for this error in VF and phasing. With my CoCo, I stripped off the outer jacket and shield and put the dielectric and inner core sections into 1/4-inch refrigerator tubing (fits perfectly). I assumed the same VF. The antenna was useless without a decoupling sleeve but works excellent with it, I used 3/4-inch copper pipe for the sleeve. I may try your twin-lead tuning method with a spare antenna and see what happens. Unfortunately I'm in Reno and surrounded by high Sierra mountains so I don't really have a clear view to the horizon in any direction. However, I'm able to pick up as far away as Fresno. I'm also picking up TIS-B in San Francisco with planes that are as low as 2,000 feet but that's a whole nuther story

[trigger]

SUCCESS!!!! - 450 kms+ : The result of 23 hrs trial run of 4-elements, 124 mm element-length, open top CoCo
Cable used: RG6, Foamed Polyethylene (FPE) insulation, VF=82%
Element length = wavelength x VF/(1+VF) = 275 mm x 0.82/(1+0.82) = 124 mm

I went back over this thread and found where I got my value of 124mm

[abcd567]

Originally Posted by abcd567
SUCCESS!!!! - 450 kms+ : The result of 23 hrs trial run of 4-elements, 124 mm element-length, open top CoCo
Cable used: RG6, Foamed Polyethylene (FPE) insulation, VF=82%
Element length = wavelength x VF/(1+VF) = 275 mm x 0.82/(1+0.82) = 124 mm

I went back over this thread and found where I got my value of 124mm
I went back over this thread and found where I got my value of 124mm

That was a failed experiment. The extraordinary results were due to exceptionally good atmospheric & weather conditions on that day. Repeat trials by me, you and HermanZA gave poor results. That is why I run the 4-element+twinlead's trial run on 4 different days for 24 hrs, and results were consistant.

[abcd567]

I'm wondering how much the outer jacket brings down the VF, since you're using the outside of the shield as the actual antenna elements and not the inside of the coax.....

The Electromagnetic field of RF signal occupies the space between aircrft antenna and coco's shield. The thickness of outer jacket is a negligible fraction of this space, and VF remains VF of air, i.e. VF=1

I'm thinking the twin-lead tuning method works because the VF is not 83 percent anymore and you're actually correcting for this error in VF and phasing......

The RF signal's phase angle varies along the length of transmission line. A specific phase shift (based on antenna's complex impedance Ra+jXa & receiver's impedance 75+j0) is required to match the two impedances. This phase shift is obtained by inserting a correct length of transmission line between antenna & feed line.

Since both the twin-lead and coax are transmission lines, a piece of coax can also be inserted in place of twin-lead.

I have decided to use twin-lead with push-pin construction, because it's length can be easily adjusted by pulling-out or pushing-in the feeder coax. Once the optimum length is set, the feeder & coco's lower element can be fixed in position by tying them to a piece of garden bamboo stick by means of cable ties.

If I would have used a piece of coax instead, then during adjustments, every time I had to reduce length, I had to cut it few millimeters. If I had to increase length, I had to replace it by a longer piece of coax. This makes adjustments very difficult

Another disadvantage of using piece of coax for impedance match is that it's shield acts as antenna element also. This disturbs the radiation pattern and gain to some extent.

Twin-lead's both wires receive RF signal, but these being in the same direction & phase, and equal in magnitude, cancel each other. Hence twin-lead does not affect radiation pattern & gain.

[abcd567]

.......The antenna was useless without a decoupling sleeve but works excellent with it, I used 3/4-inch copper pipe for the sleeve. I may try your twin-lead tuning method with a spare antenna and see what happens......

Since twin-lead and decoupler sleeve perform two different functions, try following:
(1) only twin-lead
(2) only decoupler sleeve
(3) both - decoupler sleeve + twin-lead between feeder coax & sleeve.

Using both the impedance matching & decoupling the unbalanced sheath current should logically give more improvement than using one of these alone. Hence (3) above is worth trying. But first try them one at a time.

[abcd567]

NEXT PLANNED EXPERIMENT.. maybe next week end.

Step (1): Build 8-Element CoCo made of RG6 coax, FPE insulation VF=0.83 Element Length=114 mm
Step (2): Put above CoCo on 24 hrs trial run. Repeat the 24 hrs trial run on another day(s).

Step (3): Same CoCo as above, but twin-lead impedance matching added between feed line & bottom element. Adjust length of twin-lead for best results.
Step (4): Put CoCo in step (3) on 24 hrs trial run. Repeat the 24 hrs trial run on another day(s).

Starting point: Computer simulation results from 4nec2 software:
(1) 8-Element CoCo without impedance match: SWR= 3.4, Gain = 8.5 dBi Antenna Impedance = 81.9-j102 ohms
(2) 8-Element CoCo with Twin-Lead impedance match: Twin-Lead Length 21 mm (adjust few mm for best results) , SWR= 1.02, Gain = 7.9 dBi

Twin-Lead Length for 8-Element CoCo by Smith Chart = 27.5 mm approximately (adjust few mm for best results)
DSC03427-R.jpg

[abcd567]

abcd: have you tried a 1/4 decoupling sleeve above the feeder? Looks like you're attempting something like it without the sleeve

Trial Run of 4-Element CoCo With Decoupling Sleeve

Today I removed the Twin-Lead from the 4-element coco.

I then inserted a decoupling sleeve and run a quick (1 hr) trial.

I then removed the decoupling sleeve, and ran again a quick (1 hr) trial.

The difference seems to be negligible. Please see photos & screenshots below.

no sleeve - 1 hr trial run
CoCo 4-element-quick 1 hr trial run 09 Nov2014.jpg

with sleeve - 1 hr trial run
CoCo 4-element+decoupling sleeve-quick 1 hr trial run 09 Nov2014.jpg

with sleeve
DSC03429-R.jpg

[hikeofyourlife]

What is inside your decoupling sleeve? I have a half-wave (VF adjusted) down the middle with decoupling sleeve (1/4 wave in air) connected to outer shield of bottom element. If I remove it I would have maybe 7 or 8 aircraft and with it 40-50

[hikeofyourlife]

I made another antenna with a different decoupling sleeve, this was made with a paint can cap (much wider) and aluminum tape. I punched a hole in that and put a barrel connector there to attach to the coax below and antenna (half-wave section at bottom with f connector). I had trouble maintaining good contact between the sleeve and the barrel, when it was making good contact (by wiggling it around till it shines up) I would have 30 or 40 planes then it would lose contact and I would watch the plane count drop rapidly to five or six

[abcd567]

What is inside your decoupling sleeve? I have a half-wave (VF adjusted) down the middle with decoupling sleeve (1/4 wave in air) connected to outer shield of bottom element. If I remove it I would have maybe 7 or 8 aircraft and with it 40-50

I have a 69mm piece of coax down the middle of 69mm sleeve (1/4 in air) with decoupling sleeve connected to outer shield of this 69mm element.

So I was using a wrong length of coax piece inside the sleeve.

I will replace this 69 mm piece of coax inside sleeve by 1/2 wave VF adusted piece (=114 mm), and run the trial again. The sleeve length will remain same (i.e. 1/4 in air = 69 mm)

What is the number of elements (excluding the one inside the sleeve) in your CoCo?

[hikeofyourlife]

I have a 69mm piece of coax down the middle of 69mm sleeve (1/4 in air) with decoupling sleeve connected to outer shield of this 69mm element.

So I was using a wrong length of coax piece inside the sleeve.

I will replace this 69 mm piece of coax inside sleeve by 1/2 wave VF adusted piece (=114 mm), and run the trial again. The sleeve length will remain same (i.e. 1/4 in air = 69 mm)

What is the number of elements (excluding the one inside the sleeve) in your CoCo?

A dozen. I had fewer originally and added four more with no noticeable increase in performance and it's a bit unwieldy to work on. I made up a PCB dipole antenna as well and even though it's sloppily cut with a Dremel and utility knife, it does almost as well as the CoCo!

[abcd567]

What is inside your decoupling sleeve? I have a half-wave (VF adjusted) down the middle with decoupling sleeve (1/4 wave in air) connected to outer shield of bottom element. If I remove it I would have maybe 7 or 8 aircraft and with it 40-50

I have a 69mm piece of coax down the middle of 69mm sleeve (1/4 in air) with decoupling sleeve connected to outer shield of this 69mm element.

So I was using a wrong length of coax piece inside the sleeve.

I will replace this 69 mm piece of coax inside sleeve by 1/2 wave VF adusted piece (=114 mm), and run the trial again. The sleeve length will remain same (i.e. 1/4 in air = 69 mm)

What is the number of elements (excluding the one inside the sleeve) in your CoCo?

2nd Trial Run of 4-Element CoCo With Decoupling Sleeve & 114mm (1/2 wavelength, VF corrected) piece of coax inside

Poor results!!

with sleeve & 114mm inner coax - 1 hr trial run
CoCo 4-element+decoupling sleeve-114mm inner coax-quick 1 hr trial run 10 Nov2014.jpg


with sleeve & 114mm inner coax
DSC03439-R.jpg